Mobile communication has become an indispensable part of the life of most people. With the popularization of mobile communication application, people impose higher and higher requirements on voice and data transmission. The 2nd generation mobile telecommunication system represented by the Global System for Mobile communication (GSM) and the Code Division Multiple Access (CDMA) has now exerted the transmission rate to the utmost, and the 3rd generation mobile communication system (3G) capable of transmitting data at a high speed emerges.
IP Multimedia Subsystem (IMS) is an Internet Protocol (IP) multimedia subsystem in a Wideband Code Division Multiple Access (WCDMA) defined in the 3rd Generation Partnership Project (3GPP) Release 5 (R5) standard, and is a target network for the 3G mobile network to implement VoIP and packet-based data and provide uniform multimedia services and applications.
Multiple Network Elements (NEs) of the enable logic function are introduced into the IMS architecture, including: Instant Messaging (IM), presence information service, and Extensible Mark-up Language Document Management Server (XDMS). A multi-service application architecture may be set up through such enable logic function NEs.
The presence technology may be combined with the IM service or the Push To Talk (PTT) service in the mobile data service, namely, applied to the User Equipment (UE) that supports the IM or PTT service.
The PTT enables a UE of the corresponding function to be used as a walkie-talkie, and is a half-duplex communication mode. The UE has a talk key. After pressing the talk key, the user can talk with other users or a group only if in an area covered by the operator's network regardless of the distance.
The Push-to-Talk over Cellular (PoC) is a PTT service implemented on a packet network, as defined by the Open Mobile Alliance (OMA). It uses the Voice over IP (VoIP) technology and the half duplex mode to fulfill the real-time communication requirements of users effectively at a low cost. Through the PoC service, a user may originate a PoC session to a user or a group to implement one-to-one or one-to-many sessions.
The PoC has a set of features similar to the style of the VoIP communication service and the IM service, for example, prompt of online state of a user, and online message. A user may obtain multiple voice services and multimedia services, including:                One-to-one session: A user is allowed to contact another online user selected from an address book immediately.        Temporary group: For a specific purpose, a user can select one or more pals from the online address book to make up a group temporarily and perform a group session. To perform a one-to-one conversation, the user may select any pal in the address book.        Predefined group: For a predefined group such as a sales team, the user may talk with many users in the group only through one key.        Chat group: The user may join or quit a predefined chat group or conference group.        
Therefore, groups in a PoC service fall into two types: (1) one type is the group established before a session request is originated. The information on this group is stored in the XDMS and will be used for originating a session in the future. In this circumstance, the session originator originates the session through a group ID. After receiving the request, the server obtains a group name list from the XDMS, and then invites members to join the session according to the name list; (2) another type is the temporary group, which is not established before the session request is originated. The session request sent by the session originator to the server carries a list of members to be invited into the session. After receiving the request, the server resolves the list into a name list of members to be invited, and then sends a session request to the members to be invited.
The Open Mobile Alliance (OMA) defines the PoC according to the Session Initiation Protocol (SIP) and the Real-Time Transfer Protocol (RTP) defined by the Internet Engineering Task Force (IETF).
The SIP is used as a control plane of the PoC protocol. It enables registration and authentication of a user, defines, sets up and manages session processes, supports display of the online state of other users, and sends alerts between users.
Voice packets are transmitted through the RTP protocol. Most RTP links transmit voice packets, and the remaining is based on Real-Time Transfer Control Protocol (RTCP). The RTCP provides an arbitrator of talkers in a conference, and controls the quality of RTP sessions.
A typical PoC session setup process includes six stages: setting up a data channel, registering a user, reserving a user state, inviting/accepting a session, media transmission, and session termination.
In the process of setting up a session such as group session, chat group session and temporary group session predefined in the PoC service, the session participant may choose to disclose his/her own PoC address or join the session anonymously, namely, without providing the PoC address.
In the session process, a client with the right of controlling accession to a session may expel a participant out of the session according to the PoC address of the participant. The control is exercised by a request control server.
If an expelled user reattempts to join the session, the attempt succeeds only if these two conditions are met: the session is still in progress; and the number of participants has not reached the maximum number of members to the group. Specially, an expelled user is allowed to join the session again only if the session is still in progress and the number of participants has not reached the maximum number of participants to the group.
Different PoC group sessions impose different restrictions on the PoC users that join the session. For a predefined group or a restricted chat group, only the members of the group can join the session; for an unrestricted chat group, any PoC user can join the session; for a temporary group, only the PoC users who are invited at the time of setting up the session can join the session.
By researching the previous technology, the inventor discovers following technical problems: The prior art is unable to expel an anonymous user out of the current PoC session and is unable to control reattempts to join a session made by an expelled user.
The causes for such a problem are: The client with the right of controlling user's participation into a session can expel a user only by using the PoC address of the user in a PoC session. If the user joins a session anonymously, the client is unable to obtain the PoC address of the user, and thus unable to expel the user.
Besides, if an expelled user reattempts to join a session frequently, the session participant with the right of controlling user's participation into a session has to expel the user frequently, which imposes adverse effect on the PoC session quality.